1. Field of the Invention
The present invention relates to a gradation correcting apparatus for correcting a luminance gradation of a video signal.
2. Description of the Related Art
In recent years, with the trend of demanding larger displays, the displays are required to have a gradation correcting apparatus for providing clearer images. The gradation correcting apparatus generally corrects a luminance gradation of a video signal by supplying the video signal to a nonlinear amplifier to extend a luminance distribution over the full dynamic range.
FIG. 1 illustrates a conventional gradation correcting apparatus. The illustrated gradation correcting apparatus comprises a histogram memory 1, a maximum correcting value calculating circuit 2, and a look-up table memory 3. The histogram memory 1 and the look-up table memory 3 are supplied with a digitized luminance signal as an input signal. The histogram memory 1, which is a memory for recording a luminance distribution of the input luminance signal, has a storage area for each of a plurality of preset luminance levels. Each storage area is addressed by an associated luminance level to store the frequency of the luminance level. Specifically, as one pixel portion of luminance signal is supplied, the frequency stored in a storage area of the histogram memory 1 corresponding to a luminance level of the signal is incremented by one. Also, the contents stored in the histogram memory 1 is fully cleared to zero every predetermined period (every vertical scanning period or an integer multiple thereof) to record a new luminance distribution.
The maximum correcting value calculating circuit 2 comprises a histogram accumulating circuit 2a for sequentially accumulating data in the histogram memory 1 from the lowest luminance; a cumulative histogram memory 2b for storing the result of the accumulating memory 2a; and a normalization processing circuit 2c for normalizing respective data based on data stored in the cumulative histogram memory 2b such that the largest cumulative frequency is a maximum value for an output luminance signal. The cumulative histogram memory 2b, similar to the histogram memory 1, has a storage area for each of a plurality of luminance levels of a luminance signal, which is addressed by an associated luminance level for storing the frequency thereof.
The look-up table memory 3 stores a normalized version of data stored in the cumulative histogram memory 2b. The look-up table memory 3 is addressed by a luminance level of an input luminance signal, and outputs a luminance level stored in an addressed storage area as a normalized level.
FIGS. 2A to 2C show a luminance converting operation performed by the conventional luminance correcting apparatus as represented by waveforms. For every pixel of an input luminance signal, an address in the histogram memory 1 is specified, and the value stored in a storage area at the specified address is incremented by one. Assume that the frequencies of luminance levels of the input luminance signal for a predetermined period have been detected as shown in FIG. 2A. Here, for facilitating the understanding, assume that the frequencies of luminance levels Y150, Y160, Y170, Y180, Y190, Y200, Y210 have been detected in the histogram memory 1, where a relationship Y150 less than Y160 less than Y170 less than Y180 less than Y190 less than Y200 less than Y210 is established. When the frequencies of the luminance levels Y150, Y160, Y170, Y180, Y190, Y200, Y210 for a predetermined period are 1, 3, 5, 7, 5, 3, 1, the cumulative frequencies are found to be 1, 4, 9, 16, 21, 24, 25 for the luminance levels Y150, Y160, Y170, Y180, Y190, Y200, Y210. In other words, the cumulative frequency becomes larger as the luminance level is higher, as shown in FIG. 2B. A normalizing coefficient is calculated in the normalization processing circuit 2c such that a maximum value of the cumulative frequencies is a maximum value of the output luminance levels. The normalizing coefficient is multiplied by each data in the histogram memory 1, and the results of the multiplications are stored in corresponding storage areas in the look-up table memory 3. The relationship between an input luminance level and an output luminance level in the look-up table memory 3 is represented as shown in FIG. 2C. By passing the input luminance signal through the look-up table memory 3, a gradation corrected luminance signal is output.
In the conventional gradation correcting apparatus as described above, however, since the luminance distribution is extended over the full dynamic range, such gradation correcting processing, if performed on a video such as a television moving image having a partial luminance distribution, would result in the production of a strange image far different from an actual image.
It is therefore an object of the present invention to provide a gradation correcting apparatus which is capable of preventing a luminance distribution from excessively extending over the dynamic range to accomplish an optimal gradation correction in accordance with the partiality of the luminance distribution of a particular video signal.
A gradation correcting apparatus according to the present invention includes a histogram memory for creating frequency data for each luminance level of an input luminance signal every predetermined period to store the data therein; an extension correcting table memory for creating a corrected luminance level for each of a plurality of luminance levels based on the frequency data in the histogram memory such that a luminance distribution of the input luminance signal extends over a predetermined dynamic range, and for storing the corrected luminance levels therein; means for calculating a variance value for the luminance distribution in accordance with the frequency data in the histogram memory to set a mixture ratio based on the variance value; mixing means for mixing each of the plurality of luminance levels and the corrected luminance level in the extension correcting table memory corresponding thereto in the mixture ratio; and a look-up table memory for storing the mixed level generated by the mixing means as table data corresponding to each of a plurality of luminance levels, wherein the level data corresponding to the luminance level of the input luminance signal is read from the look-up table memory, and output as a gradation corrected luminance signal.